Improvement of Electrochemical Water Oxidation by Fine‐Tuning the Structure of Tetradentate N4 Ligands of Molecular Copper Catalysts

• Published in: ChemSusChem 2017-11, Vol.10 (22), p.4581-4588
• Main Authors: Shen, Junyu, Wang, Mei, Gao, Jinsuo, Han, Hongxian, Liu, Hong, Sun, Licheng
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Chemical reactions; chemical structure; chemistry; Control stability; Controlled potential electrolysis; Coordination Complexes; coordination compound; Coordination compounds; Copper; Copper compounds; electrocatalysis; electrochemical analysis; Electrochemical Techniques; Electrolysis; Evolution; Experimental evidence; Faradaic efficiencies; homogeneous catalysis; ligand; Ligands; Molecular Structure; N ligands; Oxidation; oxidation reduction reaction; Oxidation-Reduction; oxygen; Phosphate buffer solutions; Possible mechanisms; procedures; Rate constants; Tuning; water; water oxidation
• ISSN: 1864-5631; 1864-564X; 1864-564X
• DOI: 10.1002/cssc.201701458

Two copper complexes, [(L1)Cu(OH2)](BF4)2 [1; L1=N,N′‐dimethyl‐N,N′‐bis(pyridin‐2‐ylmethyl)‐1,2‐diaminoethane] and [(L2)Cu(OH2)](BF4)2 [2, L2=2,7‐bis(2‐pyridyl)‐3,6‐diaza‐2,6‐octadiene], were prepared as molecular water oxidation catalysts. Complex 1 displayed an overpotential (η) of 1.07 V at 1 mA cm−2 and an observed rate constant (kobs) of 13.5 s−1 at η 1.0 V in pH 9.0 phosphate buffer solution, whereas 2 exhibited a significantly smaller η (0.70 V) to reach 1 mA cm−2 and a higher kobs (50.4 s−1) than 1 under identical test conditions. Additionally, 2 displayed better stability than 1 in controlled potential electrolysis experiments with a faradaic efficiency of 94 % for O2 evolution at 1.58 V, when a casing tube was used for the Pt cathode. A possible mechanism for 1‐ and 2‐catalyzed O2 evolution reactions is discussed based on the experimental evidence. These comparative results indicate that fine‐tuning the structures of tetradentate N4 ligands can bring about significant change in the performance of copper complexes for electrochemical water oxidation. Join the 4 N legion! An enhancement of the activity and a significant decrease of the overpotential for electrochemical water oxidation with a molecular copper catalyst were simultaneously attained by changing the tetradentate N4 ligand from a diamine–dipyridine to a diimine–dipyridine.